The present invention relates generally to a control system for an internal combustion engine. More specifically, the invention relates to a control system for an internal combustion engine which can attain ultimate purification performance of a catalytic converter and prevent degradation of drivablity of a vehicle or so forth.
In viewpoint of environment protection, restriction for various components in an automotive exhaust gas is getting more and more strict. In response to this, variety of means for efficiently purifying the exhaust gas have been proposed. Particularly, when the exhaust gas is purified by means of a catalytic converter, it is an important problem how to maximize purifying action of the catalytic converter. In the prior art, such as JP-A-62-203946 and JP-A-2-271046, there has been proposed means utilizing a phenomenon of improving purification rate of the catalytic converter by varying an air/fuel ratio across a stoichiometric air/fuel ratio.
Morcover, JP-A-200802 has proposed an apparatus for adjusting a deviation of air/fuel ratio in accordance with an operation condition of vehicles and a deterioration of catalyt.
On the other hand, in order to utilize the foregoing phenomenon, the foregoing prior arts take a measure to provide fine oscillation of a fuel supply amount to the internal combustion engine. However, as shown in FIGS. 7 and 9, while there are an oscillation amplitude and an oscillation period suitable for improving purification rate of the catalytic converter in such fine oscillation, it inherently cause fluctuation of a torque generated by the internal combustion engine. Therefore, so as not to sacrifice drivability, application of ideal oscillation period and oscillation amplitude for the catalytic converter has to be given up. Also, under a condition where the catalytic converter is not sufficiently activated or purification performance of the catalytic converter is degraded, if the oscillation period and the oscillation amplitude of the fuel supply amount suitable in the condition where the catalytic converter can act with sufficient purification performance, the catalytic converter may not completely purify the exhaust gas to make the composition of the exhaust gas worse.